Efforts to reduce sulfur emissions in the gaseous products from combustion of a sulfur-containing fuel have been made in varying directions. Some processes attempt to reduce or eliminate the sulfur in the fuel prior to its combustion. Other processes propose the addition of compounds to the combustion zone which will in some manner change the nature of the sulfur compounds such that they may be more readily removed from the combustion products. And yet other processes remove sulfur compounds from the gaseous combustion products by chemical reaction.
U.S. Pat. No. 4,185,080, issued Jan. 22, 1980 to Rechmeier discloses a combustion gas desulfurization process wherein a powdered sorbent such as limestone or dolomite is added to a combustion zone, and a portion of the solids collected from the flue gas is reactivated and can be returned for injection into or downstream of the combustion zone.
The approach of chemical treatment to effect flue gas desulfurization can be further subdivided into wet scrubbing wherein a solution or suspension of reagent both enters and leaves the flue gas contacting zone in liquid state, spray drying wherein a solution or suspension of reagent enters the flue gas contacting zone in liquid state but is dried to produce a powdered solid leaving the contacting zone, and dry treatment wherein the treating reagent is a solid state powder both entering and leaving the contacting zone.
U.S. Pat. No. 4,350,670, issued Sept. 21, 1982 to Matsuda et al discloses a dry treatment of flue gas containing dust and sulfur dioxide by contacting the flue gas with an oxidizing catalyst to form sulfur trioxide, which reacts with components of the dust to form a sulfate; if there is insufficient dust to absorb the sulfur trioxide, fine particles of alkaline earth oxides or carbonates can be added.
Finally, U.S. Pat. 4,330,512, issued May 18, 1982, to Lindstrom discloses treatment of flue gas by flowing it upwardly through a chimney in countercurrent contact with a shower of sorbent-coated balls.
Current thinking seems to be that no one of the above-discussed strategies is the unique answer to the sulfur emission problem, either for new installations or for retrofit on an existing installation. Rather, numerous site-specific factors such as proximity to reagent source, space availability, and extent of sulfur removal required, must enter into the selection at each plant.